Perpendicular magnetic anisotropy free layers with iron insertion and oxide interfaces for spin transfer torque magnetic random access memory

ABSTRACT

A method of making a spin-torque transfer magnetic random access memory device (STT MRAM) device includes forming a tunnel barrier layer on a reference layer; forming a free layer on the tunnel barrier layer, the free layer comprising a cobalt iron boron (CoFeB) alloy layer and an iron (Fe) layer; and performing a sputtering process to form a metal oxide layer on the Fe layer.

BACKGROUND

The present invention generally relates to spin-transfer torque magneticrandom access memory (STT MRAM) devices, and more specifically toperpendicular magnetic anisotropy (PMA) materials in STT MRAM devices.

A STT MRAM device is a type of solid state, non-volatile memory devicethat uses tunneling magnetoresistance (TMR or MR) to store information.MRAM includes an electrically connected array of magnetoresistive memoryelements, referred to as magnetic tunnel junctions (MTJs). Each MTJincludes a free layer and fixed/reference layer that each include amagnetic material. The free and fixed/reference layers are separated bya non-magnetic insulating tunnel barrier. The free layer and thereference layer are magnetically decoupled by the tunnel barrier. Thefree layer has a variable magnetization direction, and the referencelayer has an invariable magnetization direction.

The MTJ stores information by switching the magnetization state of thefree layer. When the free layer's magnetization direction is parallel tothe reference layer's magnetization direction, the MTJ is in a lowresistance state. Conversely, when the free layer's magnetizationdirection is antiparallel to the reference layer's magnetizationdirection, the MTJ is in a high resistance state. The difference inresistance of the MTJ indicates a logical ‘1’ or ‘0’, thereby storing abit of information. The TMR of an MTJ determines the difference inresistance between the high and low resistance states. A relatively highdifference between the high and low resistance states facilitates readoperations in the MRAM.

The magnetization direction of the free layer may be changed by aspin-transfer torque (STT) switched write method, in which a writecurrent is applied in a direction perpendicular to the film plane of themagnetic films forming the MTJ. The write current transfers spin angularmomentum to the free layer which creates a torque to change (or reverse)the free layer's magnetization direction. During STT magnetizationreversal, the write current for magnetization reversal is determined bythe current density. As the surface area of the the MTJ becomes smaller,the write current for reversing the free layer's magnetization becomessmaller. Therefore, if writing is performed with fixed current density,the necessary write current becomes smaller as the MTJ size becomessmaller.

Compared to MTJs with in-plane magnetic anisotropy, layers withperpendicular magnetic anisotropy (PMA) can lower the necessary writecurrent density. Thus, PMA materials lower the total write current used.

SUMMARY

In one embodiment of the present invention, a method of making aspin-torque transfer magnetic random access memory device (STT MRAM)device includes forming a tunnel barrier layer on a reference layer;forming a free layer on the tunnel barrier layer, the free layercomprising a cobalt iron boron (CoFeB) alloy layer and an iron (Fe)layer; and performing a sputtering process to form a metal oxide layeron the Fe layer.

In another embodiment, a method of making a STT MRAM device includesforming a tunnel barrier layer on a reference layer; forming a freelayer on the tunnel barrier layer, the free layer comprising a magneticlayer, a CoFeB alloy layer, a spacer layer between the magnetic layerand the CoFeB alloy layer, and an Fe layer on the CoFeB alloy layer; andperforming a sputtering process to form a metal oxide layer on the freelayer.

Yet, in another embodiment, a method of making a STT MRAM includesforming a tunnel barrier layer on a reference layer; forming a freelayer on the tunnel barrier layer, the free layer comprising a magneticlayer, a CoFeB alloy layer, a spacer layer between the magnetic layerand second CoFeB alloy layers, and an Fe layer disposed on the secondCoFeB alloy layer; and performing a sputtering process to form a metaloxide layer on the free layer; wherein sputtering process comprises RFsputtering a metal oxide onto the Fe layer under a pressure in a rangefrom about 0.1 to about 10 milli-Torr (mTorr).

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIGS. 1-3 illustrate various embodiments of STT MRAM devices accordingto the present invention, in which:

FIG. 1 illustrates a cross-sectional side view of a STT MRAM deviceutilizing CoFeB layers with PMA;

FIG. 2 illustrates a cross-sectional side view of a STT MRAM device witha free layer including an CoFeB layer and an Fe layer;

FIG. 3 illustrates a cross-sectional side view of a STT MRAM device witha free layer including a magnetic layer, a spacer layer, a CoFeB layer,and an Fe layer;

FIG. 4 is a flow diagram illustrating a method of forming a STT MRAMdevice;

FIG. 5A is a graph comparing switching efficiency (E_(b)/I_(c0)) in STTMRAM devices with free layers including CoFeB alone and a combination ofCoFeB and Fe layers; and

FIG. 5B is a graph comparing energy per bit (E_(b)) in STT MRAM deviceswith free layers including CoFeB alone and a combination of CoFeB and Felayers.

DETAILED DESCRIPTION

As stated above, the present invention relates to STT MRAM devices, andmore specifically to PMA materials in STT MRAM devices. It is noted thatlike reference numerals refer to like elements across differentembodiments.

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

As used herein, the articles “a” and “an” preceding an element orcomponent are intended to be nonrestrictive regarding the number ofinstances (i.e. occurrences) of the element or component. Therefore, “a”or “an” should be read to include one or at least one, and the singularword form of the element or component also includes the plural unlessthe number is obviously meant to be singular.

As used herein, the terms “invention” or “present invention” arenon-limiting terms and not intended to refer to any single aspect of theparticular invention but encompass all possible aspects as described inthe specification and the claims.

As used herein, the term “about” modifying the quantity of aningredient, component, or reactant of the invention employed refers tovariation in the numerical quantity that can occur, for example, throughtypical measuring and liquid handling procedures used for makingconcentrates or solutions. Furthermore, variation can occur frominadvertent error in measuring procedures, differences in themanufacture, source, or purity of the ingredients employed to make thecompositions or carry out the methods, and the like. In one aspect, theterm “about” means within 10% of the reported numerical value. Inanother aspect, the term “about” means within 5% of the reportednumerical value. Yet, in another aspect, the term “about” means within10, 9, 8, 7, 6, 5, 4, 3, 2, or 1% of the reported numerical value.

As used herein, the terms “atomic percent,” “atomic %” and “at. %” meanthe percentage of atoms of a pure substance divided by the total numberof atoms of a compound or composition, multiplied by 100.

As used herein, the term “magnetic anisotropy” means the magnetizationprefers to orient in a particular direction.

As used herein, the terms “perpendicular magnetic anisotropy” and “PMA”mean the magnetization prefers to orient perpendicular to the xy-plane.PMA can be determined by measuring magnetic hysteresis loops in bothin-plane and out-of-plane directions.

Magnetic materials with PMA are useful for free layer applications inSTT MRAM devices. However, STT devices with magnetic materials havingboth sufficiently strong PMA at a low switching current are a challenge.

Accordingly, the present invention solves the above problem by providinga method of making a STT MRAM device with desirable magnetic materials.The methods described provide STT MRAM devices with magnetic materialshaving sufficiently strong PMA at a low switching current (e.g., atleast 50% higher switching efficiency compared to STT MRAM devices withfree layers of CoFeB alone).

In particular, the inventive methods utilize sputtering processes toform an oxide cap over the free layer. The sputtering processes provideadvantages of increased control over the oxide layer thickness and,therefore, improved control of the junction resistance-area (RA)product, as well as the distribution of the RA in patterned devices.Compared to initially depositing a metal layer and then oxidizing themetal layer by an oxidation process, as described herein, the oxide caplayer is formed by RF sputtering from an oxide target.

Turning now to the Figures, FIGS. 1-3 illustrate various embodiments ofSTT MRAM devices according to the present invention. FIG. 1 illustratesa cross-sectional side view of a STT MRAM device utilizing magneticlayers with PMA. The STT MRAM device includes a magnetic tunnel junction(MTJ) 122 over a seed layer 110. The MTJ 122 includes a reference layer120, a tunnel barrier layer 130 on the reference layer 120, and a freelayer 140 on the tunnel barrier layer 130.

The reference layer 120 and the free layer 140 are magnetic materials.The free layer 140 is described in further detail in FIGS. 2 and 3below. The reference layer 120 may include any metal or metal alloys.Non-limiting examples of suitable materials for the reference layer 120include cobalt (Co), iron (Fe), boron (B), nickel (Ni), iridium (Ir),platinum (Pt), palladium (Pd), or any combination thereof.

The thickness of the reference layer 120 is not intended to be limited.In one aspect, the thickness of the reference layer 120 is in a rangefrom about 10 nanometers (nm) to about 20 nm. In another aspect, thethickness of the reference layer 120 is in a range from about 2 nm toabout 10 nm. Yet, in another aspect, the thickness of the referencelayer 120 is about or in any range from about 2, 4, 6, 8, 10, 12, 14,16, 18, and 20 nm.

The free layer 140 is shown with double arrows to illustrate that spintorque current (or spin-polarized current) via voltage source 170 canflip the magnetic orientation of the free layer 140 to up or down asdesired. The reference layer 120 is shown with an up arrow to illustratea magnetic orientation in the up direction. To write the STT-RAM device,the voltage source 170 applies voltage such that a spin torque currentmay flip the magnetic orientation of the free magnetic layer 140 asdesired. When the magnetic orientations of the free layer 140 and thereference layer 120 are parallel (i.e., pointing in the same direction),the resistance of the MTJ 122 is low (e.g., representing logic 0). Whenthe magnetic orientations of the free layer 140 and the reference layer120 are antiparallel (i.e., pointing in opposite directions), theresistance of the MTJ 122 is high (e.g., representing a logic 1).

One non-limiting example of a suitable material for the tunnel barrierlayer 130 includes magnesium oxide (MgO). Any insulating material may beused in the tunnel barrier layer 130. The thickness of the tunnelbarrier layer 130 is not intended to be limited. In one aspect, thethickness of the tunnel barrier layer 130 is in a range from about 0.5nm to about 2 nm.

The seed layer 110 may include one or more different materials,depending on the composition of the reference layer 130, to grow thereference layer 120. Non-limiting examples of suitable materials for theseed layer 110 include NiCr, Ta, TaN, Pt, Pd, Ru, Ir, or any combinationthereof. The thickness of the seed layer 110 is not intended to belimited. In one aspect, the thickness of the seed layer 110 is in arange from about 5 nm to about 10 nm. In another aspect, the thicknessof the seed layer 110 is in a range from about 1 nm to about 5 nm. Yet,in another aspect, the thickness of the seed layer 110 is about or inany range from about 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 nm.

A metal oxide layer 150 is formed on the free layer 140. The metal oxidelayer 150 is formed by a sputtering process. In one aspect, thesputtering process is a radio frequency (RF) sputtering process. DuringRF sputtering, a metal oxide is sputtered onto the free layer 140.Non-limiting examples of suitable metal oxides for forming the metaloxide layer 140 include MgO, tantalum oxide (TaOx), titanium oxide(TiOx), aluminum oxide (AlOx), magnesium titanium oxide (MgTiOx), or anycombination thereof. The thickness of the metal oxide layer 150 is notintended to be limited. In one aspect, the thickness of the metal oxidelayer 150 is in a range from about 0.2 to about 2 nm. Yet, in anotheraspect, the thickness of the metal oxide layer 150 is about or in anyrange from about 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0nm.

RF sputtering is performed under a pressure in a range from about 0.1mTorr to about 10 mTorr. The deposition rate of the oxide cap materialis controlled in a range from about 0.0005 nm/second to 0.005 nm/second.The low sputter rate provides precise control of the cap oxide layerthickness. The oxide material sputtered from the oxide target can becontrolled provide the right stoichiometry without excessive oxygen oroxygen deficiency. Such control minimizes the interaction between thefree layer material and oxide cap by taking advantage of the interfaceanisotropy. In contrast, when the oxide cap is formed by metaldeposition followed by subsequent oxidation, it is difficult to controlthe oxidation process to provide stoichiometric oxide and to provideoxygen stops exactly at the free layer interface. As a result, the freelayer can be easily oxidized and causes high RA, which is unfavorable.

Cap layer 160 is formed over the metal oxide layer 150. Non-limitingexamples of suitable materials for the metal oxide layer 150 includeruthenium (Ru), Pd, Pd, Pt, Ta, titanium nitride (TiN), or anycombination thereof. The thickness of the cap layer 160 is not intendedto be limited. In one aspect, the thickness of the cap layer 160 is in arange from about 1 nm to about 10 nm.

The seed layer 110, reference layer 120, tunnel barrier layer 130, freelayer, and cap layer 160 may be formed by any suitable depositionprocesses. Non-limiting examples of suitable deposition processesinclude physical vapor deposition (PVD), chemical vapor deposition(CVD), high density plasma CVD (HDP CVD), epitaxial growth, or othersuitable deposition processes.

FIG. 2 illustrates a cross-sectional side view of a STT MRAM device witha free layer 140 including a CoFeB alloy layer 210 and an Fe layer 220.The STT MRAM device includes a MTJ 122 over a seed layer 110. The MTJ122 includes a reference layer 120, a tunnel barrier layer 130 on thereference layer 120, and a free layer 140 on the tunnel barrier layer130. The free layer 140 includes a CoFeB alloy layer 210 and an Fe layer220 on the CoFeB alloy layer 210. The CoFeB alloy layer 210 and the Felayer 220 are discrete layers that are strongly ferromagneticallycoupled and switch as a single entity under spin torque currents. The Felayer 220 substantially improves the PMA of the free layer 140.

The CoFeB alloy layer 210 includes boron (B) in amount in a range fromabout 5 to about 50 at. %. In another aspect, the CoFeB layer 210include boron in an amount in a range from about 20 to about 30 at. %.Yet, in another aspect, the CoFeB alloy layer 210 includes boron in anamount about or in any range from about 5, 10, 15, 20, 25, 30, 35, 40,45, and 50 at. %.

The CoFeB alloy layer 210 includes iron (Fe) in amount in a range fromabout 20 to about 80 at. %. In another aspect, the CoFeB layer 210include iron in an amount in a range from about 20 to about 60 at. %.Yet, in another aspect, the CoFeB alloy layer 210 includes iron in anamount about or in any range from about 20, 25, 30, 35, 40, 45, 50, 55,60, 65, 70, 75, and 80 at. %.

The CoFeB alloy layer 210 includes cobalt (Co) in amount in a range fromabout 10 to about 50 at. %. In another aspect, the CoFeB layer 210include cobalt in an amount in a range from about 20 to about 30 at. %.Yet, in another aspect, the CoFeB alloy layer 210 includes cobalt in anamount about or in any range from about 10, 15, 20, 25, 30, 35, 40, 45,and 50 at. %.

The thickness of the CoFeB alloy layer 210 is not intended to belimited. In one aspect, the thickness of the CoFeB alloy layer 210 is ina range from about 0.2 nm to about 3 nm. In another aspect, thethickness of the CoFeB alloy layer 210 is in a range from about 0.5 toabout 2 nm. Yet, in another aspect, the thickness of the CoFeB alloylayer 210 is about or in any range from about 0.2, 0.4, 0.6, 0.8, 1.0,1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6, 2.8, and 3 nm.

In one aspect, the Fe layer 220 includes at least 98 at. % Fe. Inanother aspect, the Fe layer 220 includes at least 99 at. % Fe. Yet, inanother aspect, the Fe layer 220 is substantially pure Fe. The Fe layer220 may include other additional metals or non-metals. The thickness ofthe Fe layer 220 is not intended to be limited. In one aspect, thethickness of the Fe layer 220 is in a range from about 0.2 to about 2nm. In another aspect, the thickness of the Fe layer 220 is in a rangefrom about 0.2 to about 1.5 nm. Yet, in another aspect, the thickness ofthe Fe layer 220 is about or in any range from about 0.2, 0.4, 0.6, 0.8,1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 nm.

The CoFeB alloy layer 210 and the Fe layer 220 may be formed by anysuitable deposition processes. Non-limiting examples of suitabledeposition processes include PVD, CVD, and HDP CVD.

The metal oxide layer 150 is formed by sputtering (e.g., RF sputtering)of a metal oxide onto the Fe layer 220 as described above for FIG. 1.The cap layer 160 is formed on the metal oxide layer 150.

FIG. 3 illustrates a cross-sectional side view of a STT MRAM device witha free layer 140 including a magnetic layer 310, a spacer layer 320, aCoFeB layer 210, and an Fe layer 220. The STT MRAM device includes a MTJ122 over a seed layer 110. The MTJ 122 includes a reference layer 120, atunnel barrier layer 130 on the reference layer 120, and a free layer140 on the tunnel barrier layer 130. The free layer 140 includes amagnetic layer 310, a CoFeB alloy layer 210, a spacer layer 320 betweenthe magnetic layer 310 and the CoFeB alloy layer 210, and a Fe layer 220on the CoFeB layer 210.

The magnetic layer 310 and the CoFeB alloy layer 210, which is alsomagnetic, are ferromagnetically coupled through the spacer layer 320.When a voltage source (see FIG. 1) generates the spin torque current(spin polarized current), the magnetic orientations (maintained in thesame direction with respect to one another) of the magnetic layer 310and the CoFeB alloy layer 210 are both flipped in the same directionaccording to the angular momentum of the spin torque current.Accordingly, when the free layer 140 is parallel to the reference layer120, the resistance is low and the logic state is 0. When the free layer140 is antiparallel to the reference layer 120, the resistance is highand the logic state is 1.

Non-limiting examples of suitable materials for the magnetic layer 310include cobalt (Co), iron (Fe), boron (B), nickel (Ni), or anycombination thereof. The thickness of the magnetic layer 310 is notintended to be limited. In one aspect, the thickness of the magneticlayer 310 is in a range from about 0.2 nm to about 2 nm. In anotheraspect, the thickness of the magnetic layer 310 is in a range from about0.5 to about 1 nm. Yet, in another aspect, the thickness of the magneticlayer 310 is about or in any range from about 0.2, 0.4, 0.6, 0.8, 1.0,1.2, 1.4, 1.6, 1.8, and 2.0.

Non-limiting examples of suitable materials for the spacer layer 320include tantalum (Ta), titanium (Ti), titanium nitride (TiN), aluminum(Al), magnesium (Mg), titanium magnesium (TiMg), tantalum magnesium(TaMg), or any combination thereof. The thickness of the spacer layer320 is not intended to be limited. In one aspect, the thickness of thespacer layer 320 is in a range from about 0.1 to about 1 nm. In anotheraspect, the thickness of the spacer layer 320 is in a range from about0.2 to about 0.5 nm. Yet, in another aspect, the thickness of the spacerlayer 320 is about or in any range from about 0.1, 0.2, 0.3, 0.4, 0.6,0.6, 0.7, 0.8, 0.9, and 1.0 nm.

The metal oxide layer 150 is formed by sputtering (e.g., RF sputtering)of a metal oxide onto the Fe layer 220 as described above for FIG. 1.The cap layer 160 is formed on the metal oxide layer 150.

The magnetic layer 310 and the spacer layer 320 may be formed by anysuitable deposition processes. Non-limiting examples of suitabledeposition processes include PVD, CVD, and HDP CVD.

FIG. 4 is a flow diagram illustrating a method of forming a STT MRAMdevice (see also, FIGS. 1-3). In box 410, the method includes forming atunnel barrier layer on a reference layer. In box 420, a free layer isformed on the tunnel barrier layer. The free layer includes a CoFeBalloy layer and an Fe layer. In box 430, a sputtering process is used toform a metal oxide layer on the Fe layer. The methods described in boxes410, 420, and 430 are described in further detail above for FIGS. 1-3.

EXAMPLE

FIG. 5A is a graph comparing switching efficiency (E_(b)/I_(c0)) in STTMRAM devices with free layers including CoFeB alone (510) and thecombination of CoFeB and Fe (520). As shown, switching efficiency wasimproved by 50% in the CoFeB/Fe free layer device (520 compared to 510)at a given device size (see FIG. 1B). FIG. 5B is a graph comparingenergy per bit (E_(b)) in STT MRAM devices with free layers includingCoFeB alone (530) and the combination of CoFeB and Fe (540). As shown,both devices demonstrated similar energy per bit.

The above described STT MRAM devices and methods provide variousadvantages. The devices with free layers with a CoFeB and Fe layercombination and a metal oxide over the free layer demonstratesufficiently strong PMA at a low switching current (e.g., at least 50%higher switching efficiency compared to STT MRAM devices with freelayers of CoFeB alone). The method of making the devices, includingforming a metal oxide layer over the free layer by sputtering providesadvantages of increased control over the oxide layer thickness and,therefore, improved control of the junction resistance-area (RA)product, as well as the distribution of the RA in patterned devices.Compared to initially depositing a metal layer and then oxidizing themetal layer by an oxidation process, as described herein, the oxide caplayer is formed by RF sputtering from an oxide target.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, element components,and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

The flow diagrams depicted herein are just one example. There may bemany variations to this diagram or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order or steps maybe added, deleted or modified. All of these variations are considered apart of the claimed invention.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method of making a STT MRAM device, the methodcomprising: forming a tunnel barrier layer on a reference layer; forminga free layer on the tunnel barrier layer, the free layer comprising amagnetic layer, a CoFeB alloy layer, a spacer layer between the magneticlayer and the CoFeB alloy layer, and an Fe layer on the CoFeB alloylayer; and performing a sputtering process to form a metal oxide layeron the free layer.
 2. The method of claim 1, wherein the CoFeB alloylayer comprises boron (B) in an amount in a range from about 20 to about30 at. %.
 3. The method of claim 1, wherein the CoFeB alloy layercomprises Fe in an amount in a range from about 20 to about 60 at. %. 4.The method of claim 1, wherein the CoFeB alloy layer comprises cobalt(Co) in an amount in a range from about 20 to about 40 at. %.
 5. Themethod of claim 1, wherein the sputtering process is RF sputtering. 6.The method of claim 5, wherein the RF sputtering comprises sputtering ametal oxide onto the CoFeB alloy layer.
 7. The method of claim 6,wherein the metal oxide is MgO, tantalum oxide (TaOx), titanium oxide(TiOx), aluminum oxide (AlOx), magnesium titanium oxide (MgTiOx), or anycombination thereof.
 8. A method of making a STT MRAM device, the methodcomprising: forming a tunnel barrier layer on a reference layer; forminga free layer on the tunnel barrier layer, the free layer comprising amagnetic layer, a CoFeB alloy layer, a spacer layer between the magneticlayer and the CoFeB alloy layers, and an Fe layer disposed on the CoFeBalloy layer; and performing a sputtering process to form a metal oxidelayer on the free layer; wherein sputtering process comprises RFsputtering a metal oxide onto the Fe layer under a pressure in a rangefrom about 0.1 to about 10 milli-Torr (mTorr).
 9. The method of claim 8,wherein the Fe layer comprises at least 99 at. % Fe.
 10. The method ofclaim 8, wherein the free layer has a thickness in a range from about0.6 to about 6 nm.
 11. The method of claim 8, wherein the metal oxidelayer has a thickness in a range from about 0.2 to about 2 nm.
 12. Themethod of claim 8, wherein the sputtering process further comprisesdepositing the metal oxide at a deposition rate in a range from about0.0005 nm/second to 0.005 nm/second.